Update Firefox Nightly version from 29 to 30

Attempting to install firefox nightly and got a `curl` error. It appears
that firefox updated from firefox-29.0a1.en-US.mac.dmg to 30.

Issue: #106

Casks/firefox-nightly.rb

@@ -1,7 +1,7 @@
 class FirefoxNightly < Cask
-  url 'https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/firefox-29.0a1.en-US.mac.dmg'
+  url 'http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/firefox-30.0a1.en-US.mac.dmg'
   homepage 'http://nightly.mozilla.org/'
-  version '29.0a1'
+  version '30.0a1'
   no_checksum
   link 'FirefoxNightly.app'
 end